| 09 Mar 2026
Long-term temperature, oxygen and water clarity trends in Swiss lakes
Abstract. Water temperature, electrical conductivity, dissolved oxygen concentration, and water clarity are key variables routinely measured in Swiss lakes by lake monitoring programs run by cantonal environmental offices and international lake commissions. In most lakes, data is collected at bi-weekly to monthly intervals; however, the data processing pipelines remain largely manual, non-automated, and decentralized, making data access and use difficult. The Swiss Federal Office for the Environment (FOEN) consolidates and stores this information at the national level, but this process is not always done on a regular basis and the data are not directly accessible to the public. This limited accessibility restricts their use in scientific research, particularly for comparative studies across different lakes. Here, we present a harmonised dataset of temperature, electrical conductivity, dissolved oxygen and Secchi depth (a widely used proxy of water clarity) collected by cantonal offices, and research institutes in 21 large Swiss lakes and lake basins from the beginning of consistent records (between 1938 and 2001 depending on the lake) to the end of 2023. In addition, we provide consistently calculated variables including lake heat content, Schmidt stability, thermocline depth and hypolimnetic oxygen. We used the measured and calculated variables to identify long term trends in large Swiss lakes. Specifically, we investigated whether the effects of climate change and re-oligotrophication led to a coherent pattern across Switzerland or whether the responses varied from lake to lake. We found a clear warming trend in all lakes with consistently increasing mean annual surface (since ~1980) and bottom temperature (since ~2010), heat content, and stronger autumn stratification, which are consistent with expected effects of climate change. Similarly, there is a general trend towards greater water clarity, consistent with re-oligotrophication. Thermocline depth and lake oxygen concentrations showed less clear patterns. For the former, observations indicate modest shifts toward shallower thermoclines in deeper lakes and towards deeper thermoclines in shallower lakes. In contrast, deepwater oxygen concentrations show no consistent trends over recent decades, potentially reflecting opposing influences of climate change and re-oligotrophication. By publishing this data, we aim to advocate for open data policies at national and international levels, facilitating its reuse in further scientific research, and contributing to evidence-based lake management and decision-making.
Although many papers have been published about changes in lake characteristics depending on climate warming, eutrophication, oligotrophication and other (anthropogenic) drivers, the majority has focussed either on single lakes with long records or on – mostly iconic – large lakes with established data collections. In many cases, the focus was on temperature, ice cover and overturn events. Here, the authors have undertaken a huge work, first by scrolling through unpublished (handwritten?) records of Swiss commissions, cantonal offices and monitoring programs. Secondly, as different as the sources of these data are, considering sampling intervals, depth profiles and measured parameters (with their potential analytical errors) as helpful is the careful selection of parameters: which one is useful, how reliable are the measurements, how significant is the measured or calculated value (Schmidt stability, heat content, hypolimnetic volume …) to infer shifts in the lake’s characteristics. And, eventually, which common trends can be extracted from this comparison, and which ones are either counterintuitive or in contrast with observed trends.
Not surprisingly, surface temperatures are increasing in almost all lakes, and the inflection point in lakes with long-term records is around 1980, coinciding with almost all air and water temperature measurements in the Alps. It could be interesting for the authors to have a look at the paper of Niedrist et al. 2018 (Climate warming increases vertical and seasonal water temperature differences and inter-annual variability in a mountain lake. Climate change https://doi.org/10.1007/s10584-018-2328-6) who found warming trends only in certain months or seasons, a reversing of warming around 1995 and an unexplained increase in variability of temperatures and Schmidt stability. I imagine that it could be helpful to have a second look at their data.
I found Figure 4 (Theil-Sen regression slopes) quite interesting for it shows how differently the studied 21 lakes behave with respect to to surface and bottom temperatures. The values of heat content (Fig. 5) are normalized for the year 2000 in order to compare the curves of lakes of shorter time records, a clever idea for it allows to show also the changes in heat content of Lower Lake Zurich, Lake Zug, Lake Sempach and Greifensee going back to the 1950s, similarly also the shifts in Schmidt stability (Fig. 6). The summer thermocline depth (Fig. 7), although showing a slight trend to greater depths in some lakes, seems to oscillate in other lakes, thus indicating that there are individual drivers, obviously depending on air temperature, precipitation, inflow and wind.
The majority of lakes shows an increase in Secchi depth (Fig. 8) during the last ten years or so, others show and up and down or rather stable values, especially those lakes with already low transparency on the long run. The mean summer oxygen concentrations (Fig. 9) reflect – in my view – the wax and vane of eutrophication with increase until around the 1980s an a decrease thereafter. In contrast to these trends, the hypolimnetic oxygen concentration in autumn (Fig. 10) is more erratic, i.e., with little change over time, but with big differences (from 0 to 10 mg/L) between lakes.
I had some problems to understand Fig. 11, although the basic idea (projected oxygen recharge upon mixing) is clear. The authors might consider to use a different way to show the – positive and negative – excursions from the expected outcomes. Fig. 12 is more easy to understand and it shows the enormous difference between the 21 lakes, but here it might be advisable to use 4 colours instead of a combination of blue-red and plus-minus to present the results.
In the Conclusions the authors talk about decreasing phosphorus concentrations and their potential effect on water clarity. The re-oligotrophication, observed (or inferred) from oxygen concentrations and thermocline depths reminds me of a paper of Weniger & Sommaruga 2025 (Fifty-Year Trends Reveal Reversal from Recovery to Re-eutrophication and Reinforced Anoxia in a Managed Mountain Lake. Ecosystems https://doi.org/10.1007/s10021-025-01003-5) who found, on the contrary, a consistent re-eutrophication around 1995 – notwithstanding a continuous decrease in phosphorus inputs. Weniger & Sommaruga attributed this to warming, so it might be interesting to consider this also for the 21 Swiss lakes.